This invention relates to a process for preparing polyamide/polyolefin blends having superior toughness and stiffness compared with currently available blends of such polymers.
Polyamide resins (or polyamides) are well known in the art and have been used for many years, i.a., as engineering or molding resins, fiber-forming resins, and barrier resins in packaging materials. Crystalline polyamides, especially those made from short chain monomers, e.g., nylon 6 and nylon 66, are very stiff, which is desirable for molding resins, but they are sensitive to moisture. Addition of crystalline polyolefins, such as polyethylene or polypropylene reduces moisture sensitivity but suffers from incompatibility between these two polymers, so that acceptable end-use blends are not made. Further, although polyamides have good physical properties, such as tensile strength and flexural (or flex) modulus, that make these resins suitable for fabricating a variety of articles, they are not considered tough. Their notched Izod impact strength either is too low under ordinary test conditions or decreases significantly with a modest change of temperature, thickness, orientation, or notch radius. It, therefore, is conventional in the industry to toughen polyamide resins by blending them with soft, rubbery polymers which can absorb mechanical shocks without significantly impairing the tensile properties of the polyamides with which they are blended. However, because those soft polymers normally have very low stiffness (or flex moduli), addition of such polymers to thermoplastic resins normally results in blends that have lower flex moduli than the matrix resins alone, so that improvement of one property is accomplished at the sacrifice of the other.
While there is abundant art describing such blends, the most pertinent patent in this area is U.S. Pat. No. 4,174,358, to Epstein, describing blends of polyamides with various types of toughening polymers. The Epstein invention involves a process for the preparation of multiphase thermoplastic compositions comprising admixing a defined polyamide matrix resin with at least one of a number of possible polymers having a much lower tensile modulus and containing sites which are adherent to the matrix resin, and then shearing to disperse the polymer in the matrix resin to a particle size of 0.01 to 3 microns, so that the polymer adheres to the matrix resin. According to the patent specification and certain examples, the blend may be either binary or ternary. A ternary blend contains the matrix resin, a soft, adherent polymer, and a third soft polymer, which may or may not possess adherent sites; when it does not, it is a straight chain or branched polyethylene. Such blends can be made in one step, or the soft polymers can be preblended and then re-extruded with the matrix resin. Additional compositions of this type are described in Japanese Patent Publications (Kokai) 59-78256 and 59-149940 (both 1984) of Mitsui Petrochemical Industries, Ltd.
U.S. Pat. No. 4,780,505 to Mashita et al. describes a process where polypropylene grafted with maleic anhydride is added to a polyamide/polypropylene blend to improve the compatibility of the blend. This grafted polymer can be replaced by or added together with another polymer, such as a rubber grafted with maleic anhydride or an ethylene copolymer containing a maleic anhydride or glycidyl methacrylate comonomer.
A recent paper by Modic et al. in Plastics Engineering, July 1991, pp. 37-39, describes blends of nylon 66 with polypropylene and of nylon 6 with polypropylene, both with a compatibilizer-styrenic block copolymer functionalized with maleic anhydride-which have high impact strength. The paper discusses mainly the situation where polypropylene becomes the matrix and nylon the dispersed phase, although the possibility of phase inversion in the case of a large nylon/polypropylene ratio is mentioned.
It thus is known that one can make ternary blends with desirable mechanical properties by properly selecting the component polymers A, B, and C in optimum ratios and blending them under suitable conditions. Ternary blends, when properly made, can provide a more desirable balance of properties than binary blends; yet, in practice these terms "properly", "optimum" and "suitable" are very broad and undefined. Even for skilled plastics engineers and chemists, this is a rather occult art, which requires a great deal of experimentation before a satisfactory composition is obtained. Normally, the matrix polymer A is preselected according to the business need; the two other components B and C are then varied, either with respect to their characteristics or amounts, or both, and one or more plots or tables reflecting the changes of desired properties with changing compositions are made. Usually, those plots or tables show a trend of either increasing or decreasing property values as the compositions are varied. When the desired properties have been obtained, the operation is considered successful, and the blend is adopted for commercial purposes. In some cases, with a particular matrix resin, the experimenter may find that the desired properties are difficult or impossible to obtain; the experimenter then has the choice of making do with what he or she has or replacing the matrix resin with another matrix resin and repeating the series of experiments. By following the directives of the Epstein patent, one can in some cases obtain "supertough" nylon resins, which for the purpose of the present invention means that their notched Izod value is at least 10 ft.-lb/inch (about 534 J/m).
Crystalline polyolefins such as, for example, polyethylene, polypropylene, and polyisobutylene are often considered suitable polymers for blending with polyamides because they improve the polyamide toughness while reducing the stiffness of polyamides to a lesser degree than do amorphous and rubbery polyolefins. However, binary blends of polyamides with crystalline polyolefins do not show greatly improved toughness. Addition of a third component, or compatibilizer, permits a more thorough blending, resulting in a better dispersion, so that the resulting blend has considerably improved toughness. While this is known, the proper choice of the blend components as well as of the blending conditions still is to a large extent left to the experimenter who, after a number of trial runs, arrives at a satisfactory composition and process.
Generally speaking, it is very difficult to prepare a supertough polyamide with high stiffness or to even predict under what conditions or with what components such a composition could be made.
It is, therefore, highly desirable to be able to select in advance compatibilizer B for a given crystalline polyolefin C to be blended with matrix polyamide A under predetermined blending conditions in order to obtain the maximum degree of improvement of notched Izod impact strength for a given flex modulus (which can be calculated in advance from the flex moduli of the components), and to obtain in fact such improvement with a minimum amount of experimentation.